eMedicine Specialties > Infectious Diseases > Viral Infections

Norwalk Virus

Author: Todd S Wills, MD, Assistant Professor, Department of Internal Medicine, Division of Infectious Diseases, University of South Florida College of Medicine
Coauthor(s): Michelle A Jaworski, MD, Consulting Staff, Midland Orthopedic Associates
Contributor Information and Disclosures

Updated: Jun 30, 2006

Introduction

Background

Noroviruses are a group of single-stranded RNA viruses in the family Caliciviridae that cause acute gastroenteritis in humans. Norwalk virus is the prototypical strain in the genus. Norovirus infection is an important cause of acute gastroenteritis in humans and is the most frequently implicated cause of outbreaks of viral gastroenteritis worldwide. Noroviruses cause an estimated 20% of all viral gastroenteritis cases in persons older than 24 months.

Norwalk virus was first associated with gastroenteritis in 1972. It was identified by electron microscopy of stool samples that had been saved from a 1968 gastroenteritis epidemic that occurred in Norwalk, Ohio. In a 2-day period, acute gastroenteritis developed in 50% of 232 students or teachers in an elementary school. The virus initially was labeled as a small, round, structured virus, and it was named after the city in which the outbreak occurred. Recently, norovirus infections have made news as the cause of several outbreaks of gastroenteritis on cruise ships.

Pathophysiology

The viruses are transmitted via the fecal-oral route. The most common routes of infection include contact with an infected individual or contact with contaminated food and water. Noroviruses can also be spread via particles aerosolized with vomiting. The viruses are highly contagious; only 100 viral particles are likely required to establish infection. Noroviruses can survive freezing and heating temperatures of up to 140°F. Infection is characterized by damage to microvilli in the small intestine, causing malabsorption. Vomiting is related to a virus-mediated change in gastric motility and delayed gastric emptying. Notably, no histopathologic lesions are identified in the gastric mucosa of infected patients. Noroviruses do not invade the colon, therefore fecal leukocytes are typically absent and hematochezia is rare.

Recurrent infections can occur throughout life because of the diversity of norovirus strains and the lack of cross-strain or long-term immunity.

Frequency

United States

• The frequency of norovirus disease in the United States is difficult to quantify because of the lack of readily available diagnostic assays. Recent efforts to determine the etiology of diarrheal diseases in the United States have suggested that noroviruses cause up to 17% of community cases of diarrhea and 7% of cases that require physician treatment.

Among 232 outbreaks of norovirus gastroenteritis reported to the Centers for Diseases Control between 1997 and 2000, 57% were foodborne, 16% involved person-to-person spread, 3% were waterborne, and the remainder were unknown. The location of these outbreaks included restaurants (36%), nursing homes (23%), schools (13%), and resorts/cruise ships (10%). In September 2005, an outbreak of norovirus gastroenteritis affected approximately 1000 Hurricane Katrina evacuees in temporary facilities in Houston, Tex. This episode illustrates the increased risk of such outbreaks in overcrowded conditions.

International

Data regarding outbreaks in developing nations are not well quantified, but the outbreak rate in other industrial nations is similar to that of the United States.

Mortality/Morbidity

Norovirus gastroenteritis typically lasts 24-48 hours. Death is extremely rare except in those particularly vulnerable to dehydration.

Clinical

History

• Symptomatic gastroenteritis typically develops 24-48 hours after ingestion of contaminated food or water or after contact with an infected individual. Each episode is short-lived, lasting approximately 1-2 days.
• Complaints include the following:
  • Nausea and vomiting (profuse, nonbloody, nonbilious)
  • Watery diarrhea
  • Abdominal cramps
  • Headache
  • Low-grade fever
  • Myalgia

Physical

• Vital signs include low-grade fever, tachycardia, possible hypotension (if dehydrated).
• Fecal leukocytes are absent.
• Abdominal examination findings include the absence of both focal tenderness and peritoneal signs.

Causes

• Contaminated water supply
• Frequently implicated foods include salad, cake frosting, clams, oysters, and meats.
• Viral shedding
  • The virus is shed in vomitus and feces.
  • Contamination can occur through an infected water supply, undercooked foods, or improper hand washing by an infected food preparer.
  • Because of viral shedding during profuse vomiting and diarrhea, secondary transmission commonly occurs among close contacts of the infected person.
  • The virus causes infection with a low inoculum exposure (100 viral particles) and is stable with freezing or temperatures of up to 140°F.
  • Cruise ships are particularly vulnerable because of frequent changes in the passenger cohort, relative crowding, and the difficulty in performing decontamination during short periods at shore.

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